1. Field of the Invention
The present invention relates to a tuner for a television set.
2. Description of the Related Art
In a television tuner, the frequency of a desired channel of a received television signal is amplified through a tuning circuit. In this tuning circuit, a varactor diode is used. Since a range of a change in capacitor of the varactor diode is limited to a specific range, especially, a so-called band switching type tuning circuit is used in a VHF band which covers a wide frequency range from 50 MHz to 300 or 400 MHz is used. The tuning circuit is designed to be tuned to the frequency of a reception channel in each band such as a high band, a middle band, or a low band by switching the tuning circuit.
A conventional television tuner will be described below with reference to FIGS. 3 and 4. FIG. 3 shows a part of the conventional television tuner, and FIG. 4 is a format of channel-selection data for controlling an operation of the tuner shown in FIG. 3. Referring to FIG. 3, a television signal from an antenna (not shown) or a cable television signal is input to an input terminal 1 through a cable. The frequency range of the television signals is set to be a range from 500 MHz to 800 MHz in the U.S.A. The television signal input to the input terminal 1 is distributed by two. One television signal is input to a VHF receiver 2, and the other is input a UHF receiver 3. The VHF receiver 2 is constituted by a trap circuit 4 for attenuating a television intermediate frequency (IF frequency) band, a VHF tuning circuit 5, and a VHF amplification circuit 6, and the UHF receiver 3 is constituted by a UHF tuning circuit 7 and a UHF amplification circuit 8.
The television signal input to the VHF receiver 2 is amplified by the VHF amplification circuit 6 after a desired channel frequency is selected by the VHF tuning circuit 5, and then guided to a VHF mixing circuit or the like of a following stage (not shown). The other television signal of the television signals distributed by two is amplified by the UHF amplification circuit 8 after a desired channel frequency is selected by the UHF tuning circuit 7 of the UHF receiver 3 in the same manner as described above, and then guided to a UHF mixing circuit or the like of a following stage (not shown).
The operation of the VHF receiver 2 and the operation of the UHF receiver 3 are controlled by a channel-selection control circuit 11 on the basis of channel-selection data (data for specifying a selected channel) from a channel selector of a television set (not shown). More specifically, when a channel in the VHF band is selected, the channel-selection control circuit 11 supplies a power supply voltage to the VHF amplification circuit 6 to stop the supply of a power supply voltage to the UHF amplification circuit 8. On the other hand, when a channel in the UHF band is selected, a power supply voltage is supplied to the UHF amplification circuit 8 in the same manner as described above to stop the supply of a power supply voltage to the VHF amplification circuit 6. The channel-selection control circuit 11 supplies a tuning voltage to the VHF tuning circuit 5 and the UHF tuning circuit 7.
In addition, when a channel in the VHF band is selected, the VHF tuning circuit 5 is switched by the channel-selection control circuit 11 in such a manner that, depending on whether the selected channel belongs a high band or a low band, the VHF tuning circuit 5 is properly tuned to a channel frequency in these bands. In the prior art shown in FIG. 3, a 2-band-switching type tuning circuit in which a low band ranging from about 50 MHz to 130 MHz and a high band ranging from 130 MHz to 360 MHz are switched to each other by the channel-selection control circuit 11 is constituted.
In the channel-selection control circuit 11, for example, a PLLIC (integrated circuit) for television synthesizer is used. Channel-selection data from a channel selector of a television set (not shown) is input to a data input terminal SDA, and a clock signal is input to a clock input terminal CL. In addition, a voltage of 5 V for operating the channel-selection control circuit 11 is supplied to a power supply terminal VC1, and a voltage of 12 V used as the power supply voltages of the VHF amplification circuit 6 and the UHF amplification circuit 8 and also used as a control voltage for band switching in the VHF tuning circuit 5 is supplied to another power supply terminal VC2.
A format of channel-selection data input to the data input terminal SDA of the channel-selection control circuit 11, as shown in FIG. 4(A), is constituted by band switch data consisting of 4 serial bits and frequency data consisting of 15 serial bits. The band switch data is generally used to identify whether a selected channel belongs a low band or a high band. The frequency data is used to generate a tuning voltage required for tuning the VHF tuning circuit 5 or the UHF tuning circuit 7 to the frequency in the selected channel, and is data for controlling the PLL incorporated in the channel-selection control circuit 11.
For example, when 4-bit band switch data is given by 1, 0, 0, 0 as shown in FIG. 4(B), the 12-V voltage supplied to the power supply terminal VC2 is output to a switch terminal BS4 as a control voltage by the switch circuit (not shown) in the channel-selection control circuit 11. When the band switch data is given by 0, 1, 0, 0 as shown in FIG. 4(C), the voltage is output to a switch terminal BS3. When the band switch data is given by 0, 0, 1, 0 as shown in FIG. 4(D), the voltage is output to a switch terminal BS2. When the band switch data is given by 0, 0, 0, 1 as shown in FIG. 4(E), the voltage is output to a switch terminal BS1. In this prior art, in selection of a VHF low-band channel, the band switch data shown in FIG. 4(B) and output from a channel selector of a television set (not shown) is input to the data input terminal SDA; in selection of a VHF high-band channel, the band switch data shown in FIG. 4(C) is input to the data input terminal SDA; and in selection of a UHF channel, the band switch data shown in FIG. 4(D) is input to the data input terminal SDA.
The 12-V voltage supplied to the power supply terminal VC2 corresponds to a band to which the selected channel belongs. That is, the 12-V voltage is output to the switch terminal BS4 in selection of a low-band channel; the 12-V voltage is output to the switch terminal BS3 in selection of a high-band channel; and the 12-V voltage is output to the switch terminal BS2 in selection of a UHF-band channel. A tuning voltage corresponding to the selected channel is output to a terminal VT. In this prior art, since the VHF band has a two-band configuration constituted by two low and high bands, the two VHF bands and one UHF band constitute a three-band configuration. The switch terminal BS1 is not used to be an idle terminal.
The VHF tuning circuit 5 is basically constituted by varactor diodes 12 and 13 serving as tuning capacitors, tuning coils 14, 15, 16, and 17, and switch diodes 18 and 19. When a low-band channel is selected, the switch diodes 18 and 19 are reversely biased by a 12-V control voltage from the switch terminal BS4 of the channel-selection control circuit 11 to be rendered non-conductive, and a low-band tuning circuit is constituted by the three series tuning coils 14, 17, and 16 and the two varactor diodes 12 and 13. In this case, an impedance on an antenna side (to be referred to as a signal source hereinafter) is stepped up by a rate of the tuning coil 14 to the sum of the tuning coils 16 and 17, and is matched to the impedance of the VHF amplification circuit 6.
When a high-band channel is selected, the switch diodes 18 and 19 are forward-biased by a control voltage from the switch terminal BS3 of the channel-selection control circuit 11 to be rendered conductive, the tuning coils 14 and 15 are connected in parallel, and both the ends of the tuning coil 17 are short-circuited. The tuning coil 16 is connected in series with the tuning coils 14 and 15 connected in parallel, thereby constituting a high-band tuning circuit. The impedance of a signal source is stepped up by a rate of the coils 14 and 15 connected in parallel to the coil 16 to be matched to the impedance of the VHF amplification circuit 6.
In this manner, in the conventional VHF tuning circuit 5, the tuning coils 14 and 16 of the tuning coils 14, 15, 16, and 17 are commonly used in reception of a low-band signal and reception of a high-band signal, and determine a tuning frequency in a high band and a tuning frequency in a low band while interlocking with the varactor diodes 12 and 13. The impedance of the signal source is stepped up by a so-called L tap to be matched to the impedance of the VHF amplification circuit 6. On the other hand, the UHF tuning circuit 7 is constituted by a tuning coil 20 and a varactor diode 21 but does not constitute an L tap because the impedance of the UHF amplification circuit 8 in a UHF band is low so as to be matched to the impedance of the signal source without changing the impedance.
In reception of a low-band VHF signal or a high-band VHF signal, 12-V control voltages from the switch terminals BS4 and BS3 of the channel-selection control circuit 11 are supplied to the VHF amplification circuit 6 as power supply voltages through other switch diodes 22 and 23. Similarly, in reception of a UHF signal, the control voltage from the switch terminal BS2 of the channel-selection control circuit 11 is supplied to the UHF amplification circuit 8 as a power supply voltage to the UHF amplification circuit 8. A tuning voltage is applied from the tuning voltage terminal VT of the channel-selection control circuit 11 to the varactor diodes 12 and 13 of the VHF tuning circuit 5 and the varactor diode 21 of the UHF tuning circuit 7. Note that the tuning voltage is output from the tuning voltage terminal VT in such a manner that an external voltage (e.g., 33 V) applied from an external terminal VB to the terminal VT of the channel-selection control circuit 11 through a resistor 24 is controlled by the PLL circuit in the channel-selection control circuit 11.
As described above, as a conventional VHF tuning circuit, a band switching type tuning circuit is used to be tuned to a wide frequency band in the VHF band. In addition, in order to match the impedance of a signal source to the impedance of a VHF amplification circuit, the impedance of the signal source is stepped up by a tuning coil, and, at this time, some of the tuning coils is commonly used in a high band and a low band. For this reason, it is difficult to set a tuning coil which makes setting of the tuning frequency and matching of impedances compatible with each other in both the high band and the low band. A band switching operation is performed by switching some of the tuning coils by rendering a switch diode conductive or non-conductive. For this reason, a residual capacitance of the switch diode in a non-conductive state is connected to the tuning coil, and the following problem is posed. That is, an equivalent inductance including the residual capacitance changes depending on a frequency to offset both the tuning frequency and the impedance from set values.
The impedance generally increases as the frequency of an amplification circuit due to the characteristics of an amplification element such as an FET used in the amplification circuit. For this reason, Q of the tuning circuit constituted in a low-band state is higher than Q of the tuning circuit constituted in a high-band state, and Q of the tuning circuit increases as the frequency in the same band. For this reason, the frequency response characteristics (input-output characteristics to frequency) of the tuning circuit considerably change between bands and in a band in the entire VHF band. As a result, in a conventional television tuner, as indicated by solid lines in FIGS. 5(A) to 5(F), frequency characteristics are considerably changed depending on the change in tuning frequency even in the same band. More specifically, band widths at F0=100 MHz and F0=50 MHz are smaller than that at F0=130 MHz in low band, and band widths at F0=140 MHz and F0=250 MHz are smaller than that at F0=360 MHz in a high band. Although it is not apparent from FIG. 5, the band width in a low band is smaller than that in a high band.
Therefore, the frequency characteristics of the amplitude of a video output from a television set is degraded to adverse affect image quality. In addition, phase characteristics and delay characteristics in the band of a reception channel may be degraded, and the television set does not easily cope with digitalization of a television system in the future. This invention has as its object to make it easy to perform setting of the tuning frequency and matching between impedances in respective reception bands and to correct the band width which becomes small between reception bands or between reception channels in the same band to make the frequency characteristics uniform.